1. Technical Field
The present invention relates to a four branch differential transmission system (“FBD”) for use in so-called power split transmissions (“PST”), particularly for use in vehicles. Such transmissions transmit mechanical power through two parallel paths and their speed ratio may be varied in a stepless and controlled manner.
2. Description of Related Art
Conventional three branch differential transmission systems include a planetary gear train, in which a sun gear is connected to a central shaft and is positioned concentrically within an internally toothed ring gear. A number of planet gears are situated between and mesh with the sun and the ring gears. The planet gears are held on bearings on a carrier, which is rotatably mounted concentrically on the central shaft. With this typical arrangement it is possible for the sun and ring gears to rotate around the central axis. The planets are free to rotate on their own axes and are held in their relative positions by means of the carrier and are therefore able to orbit the central axis. Generally, two of the sun gear, ring gear and the carrier are connected to an input and an output shaft. This leaves one of the three unconnected. If this free branch is rotated by an additional controlled drive, the ratio of input speed to output speed can be adjusted.
This method of changing transmission ratios makes planetary gear trains especially attractive for vehicle transmissions, as automatic transmissions can be simply constructed. By coupling two planetary gear trains, an FBD can be created, whereby, with the use of two variable speed drives connected to the two branches which are not connected to the input or output, the overall transmission ratio can be adjusted over a very high speed range. U.S. Pat. No. 5,558,589 shows the practical application of such a transmission which uses several regimes for an agricultural vehicle. Two planetary gear trains are arranged in parallel to form the FBD, whereby the carrier of one gear train is connected to the ring gear of the other. The two variable speed drives constitute electric motors. Adjustment in speed of one electric motor is sufficient to change the transmission ratio of the FBD. To reach the required range of output speeds however, several transmission regimes are required, which are achieved by engagement and disengagement of several clutches.
WO94/08156 and WO94/10483 describe several multi-regime PST, which used FBD (Ravigneaux gear train). Again several double clutches and additional gearbox components are required to change regime.
GB 2363173 A shows an electrical single regime PST for automotive vehicles, whereby the two planetary trains are placed side by side. The trains are connected via the suns, which are mounted on a common shaft, and by the carrier of the first gear train being connected to the ring gear of the second. The drive input is from a flywheel, which is connected to the carrier of the first gear train, and the output is via the carrier of the second train. This arrangement has a good efficiency, as the motors are designed in such a way that one motor uses electrical energy, which is generated by the other. To a large extent, electrical energy losses can therefore be avoided.
The planetary gear trains of known FBD generally consist of two co-called “minus gearsets”, situated side by side. A minus epicyclic gearset is one which incorporates three gear elements and in which rotation of the sun gear in one direction results in rotation of the gear element with which the planet gears are in mesh in the opposite direction whilst the carrier is held stationary. In a positive gearset, the sun gear and the gear element with which the planet gears are in mesh rotate in the same direction when the carrier is held stationary. Known FBD generally comprise two ring gears, which are expensive to manufacture, especially if high precision is required, as is the case for most of these gear trains. Furthermore, it is difficult to achieve basic Ro ratios (number of teeth on the ring gear divided by the number of teeth on the sun gear) of less than −2.00 (minus gear train), as the planet gears become too small. For many applications, however, a low Ro is desirable.
EP 1279545 A2 again describes an application of a so-called Ravigneaux planetary gearset. This is a combination of a plus and a minus gear train, whereby the assembly requires only one ring gear. However, the problem of not being able to achieve Ro ratios greater than +2.00 and less than −2.00 still exists.
The object of the invention is to provide an FBD which reduces the number of costly parts and which achieves Ro ratios of less than +/−2.00.